yes, you do. You only need to announce it to the country, lolz. You are not supposed to be tax resident in more than one country, that is what the term means, but up to you if you want to make tax declarations in half a dozen of countries.
It is extremely unlikely that you are tax resident in two countries. I suggest to check the rules. In two EU countries it is basically impossible, as the amount of days to be tax resident is 185 days, and for two countries that would be 370 days. The year only has
Sorry, but I had to laugh. He probably wanted to hint at a visa based on retirement. For that you need to be 50 (and not necessarily really retired). See my other top level post.
Before you go an ED visa route, I suggest to consider a _multiple_ _entry_ tourist visa. That would give you via leaving the country and reentering and extensions of stay close to 9month of stay. It should not be a problem to get a second one afterwards, and makes no hassle in simple letting it expire. An ED visa on the other hand needs to be canceled, which means an extra visit at immigrations.
That said, regarding ED visa and my assumption from your photos, you might be interested in Self Defense (as a sport, not out of other reasons) or close combat. In Chiang Mai are several "martial arts" gyms that offer self defense, close combat or martial arts with one year ED visa. I can recommend
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EDIT: P.S. perhaps your employer gives you an option for a one year sabbatical - but I guess you considered that already, reading your answers to some comments :P
Yes, in Germany or any other country it would be much higher. And then you forget the tax brackets before you hit that point and the tax free brackets.